Method and system for supply-air re-circulation

ABSTRACT

An HVAC system includes an evaporator coil disposed between a supply air duct and a return air duct. A re-circulation duct fluidly couples the supply air duct and the return air duct. A damper is disposed in the re-circulation duct and is moveable between an open position and a closed position. A controller is operatively coupled to a variable-speed compressor, a variable-speed circulation fan, and the damper. Responsive to a determination that the variable-speed circulation fan is operating at the minimum speed and the suction pressure is above the pre-determined threshold, the controller signals the damper to move to the open position. Responsive to a determination that the variable-speed circulation fan is not operating at the minimum speed or the suction pressure is below the pre-determined threshold, the controller signals the damper to move to the closed position.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application incorporates by reference a patent applicationSer. No. 16/208,858 bearing and titled, METHOD AND SYSTEM FOR UTILIZINGA BYPASS HUMIDIFIER FOR DEHUMIDIFICATION DURING COOLING.

TECHNICAL FIELD

The present disclosure relates generally to heating, ventilation, andair conditioning (HVAC) systems and more particularly, but not by way oflimitation, to utilizing a re-circulation duct to maximize latentcapacity of an HVAC system at low circulation fan speeds.

BACKGROUND

This section provides background information to facilitate a betterunderstanding of the various aspects of the disclosure. It should beunderstood that the statements in this section of this document are tobe read in this light, and not as admissions of prior art.

HVAC systems are used to regulate environmental conditions within anenclosed space. Typically, HVAC systems have a circulation fan thatpulls air from the enclosed space through ducts and pushes the air backinto the enclosed space through additional ducts after conditioning theair (e.g., heating, cooling, humidifying, or dehumidifying the air). Todirect operation of the circulation fan and other components, HVACsystems include a controller. In addition to directing operation of theHVAC system, the controller may be used to monitor various components,(i.e. equipment) of the HVAC system to determine if the components arefunctioning properly.

SUMMARY

Various aspects of the disclosure relate to a heating, ventilation, andair conditioning (HVAC) system. The HVAC system includes an evaporatorcoil disposed between a supply air duct and a return air duct. Are-circulation duct fluidly couples the supply air duct and the returnair duct. A damper is disposed in the re-circulation duct and ismoveable between an open position and a closed position. A controller isoperatively coupled to a variable-speed compressor, a variable-speedcirculation fan, and the damper. A pressure sensor is disposed in asuction line between the evaporator coil and the variable-speedcompressor. The pressure sensor is configured to measure a refrigerantpressure in the suction line. The controller is configured to determineif the variable-speed circulation fan is operating at a minimum speedand if a suction pressure measured by the pressure sensor is above apre-determined threshold. Responsive to a determination that thevariable-speed circulation fan is operating at the minimum speed and thesuction pressure is above the pre-determined threshold, the controllersignals the damper to move to the open position. Responsive to adetermination that the variable-speed circulation fan is not operatingat the minimum speed or the suction pressure is below the pre-determinedthreshold, the controller signals the damper to move to the closedposition.

Various aspects of the disclosure relate to a heating, ventilation, andair conditioning (HVAC) system. The HVAC system includes an evaporatorcoil disposed between a supply air duct and a return air duct. Are-circulation duct fluidly couples the supply air duct and the returnair duct. A damper is disposed in the re-circulation duct and ismoveable between an open position and a closed position. A controller isoperatively coupled to a variable-speed compressor, a variable-speedcirculation fan, and the damper. A temperature sensor is disposed in thesupply air duct and is configured to measure a temperature of air in thesupply air duct. The controller is configured to determine if thevariable-speed circulation fan is operating at a minimum speed and ifthe temperature of air in the supply air duct is above a pre-determinedthreshold. Responsive to a determination that the variable-speedcirculation fan is operating at the minimum speed and the temperature ofair in the supply air duct is above the pre-determined threshold, thecontroller signals the damper to move to the open position. Responsiveto a determination that the variable-speed circulation fan is notoperating at the minimum speed or the temperature of air in the supplyair duct is below the pre-determined threshold, signaling the damper tomove to the closed position.

Various aspects of the disclosure relate to a method of utilizing are-circulation duct. The method includes determining with an HVACcontroller if a variable-speed circulation fan is operating at a minimumspeed and monitoring an operating parameter of an HVAC system. Invarious embodiments, the method includes determining if the operatingparameter of the HVAC system exceeds a pre-determined threshold.Responsive to a determination by the HVAC controller that thevariable-speed circulation fan is operating at the minimum speed and theoperating parameter exceeds the pre-determined threshold a damperdisposed in a re-circulation duct that fluidly couples a supply air ductand a return air duct is signaled with the HVAC controller to move to anopen position.

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofclaimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with standard practice in the industry, variousfeatures are not drawn to scale. In fact, the dimensions of variousfeatures may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 is a block diagram of an exemplary HVAC system;

FIG. 2 is a schematic diagram of an exemplary HVAC system having asuction pressure sensor according to aspects of the disclosure;

FIG. 3 is a schematic diagram of a re-circulation duct according toaspects of the disclosure;

FIGS. 4A-4D are graphs illustrating various performance parameters ofthe HVAC system of FIG. 2 when the HVAC system is operating in thecooling mode according to aspects of the disclosure;

FIGS. 5A-5D are graphs illustrating various performance parameters ofthe HVAC system of FIG. 2 when the HVAC system is operating in thedehumidification mode according to aspects of the disclosure;

FIG. 6 is a schematic diagram of an exemplary HVAC system having adischarge air temperature sensor according to aspects of the disclosure;

FIG. 7A-7D are graphs illustrating various performance parameters of theHVAC system of FIG. 6 according to aspects of the disclosure; and

FIG. 8 is a flow diagram of a process for utilizing a re-circulationduct according to aspects of the disclosure.

DETAILED DESCRIPTION

Various embodiments will now be described more fully with reference tothe accompanying drawings. The disclosure may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein.

HVAC systems are frequently utilized to adjust both temperature ofconditioned air as well as relative humidity of the conditioned air. Acooling capacity of an HVAC system is a combination of the HVAC system'ssensible cooling capacity and latent cooling capacity. Sensible coolingcapacity refers to an ability of the HVAC system to remove sensible heatfrom conditioned air. Latent cooling capacity refers to an ability ofthe HVAC system to remove latent heat from conditioned air. In a typicalembodiment, sensible cooling capacity and latent cooling capacity varywith environmental conditions. Sensible heat refers to heat that, whenadded to or removed from the conditioned air, results in a temperaturechange of the conditioned air. Latent heat refers to heat that, whenadded to or removed from the conditioned air, results in a phase changeof, for example, water within the conditioned air. Sensible-to-totalratio (“S/T ratio”) is a ratio of sensible heat to total heat (sensibleheat+latent heat). The lower the S/T ratio, the higher the latentcooling capacity of the HVAC system for given environmental conditions.

Sensible cooling load refers to an amount of heat that must be removedfrom the enclosed space to accomplish a desired temperature change ofthe air within the enclosed space. The sensible cooling load isreflected by a temperature within the enclosed space as read on adry-bulb thermometer. Latent cooling load refers to an amount of heatthat must be removed from the enclosed space to accomplish a desiredchange in humidity of the air within the enclosed space. The latentcooling load is reflected by a temperature within the enclosed space asread on a wet-bulb thermometer. Setpoint or temperature setpoint refersto a target temperature setting of the HVAC system as set by a user orautomatically based on a pre-defined schedule.

When there is a high sensible cooling load such as, for example, whenoutside-air temperature is significantly warmer than an inside-airtemperature setpoint, the HVAC system will continue to operate in aneffort to effectively cool and dehumidify the conditioned air. Whenthere is a low sensible cooling load but high relative humidity such as,for example, when the outside air temperature is relatively close to theinside air temperature setpoint, but the outside air is considerablymore humid than the inside air, supplemental air dehumidification isoften undertaken to avoid occupant discomfort.

An existing approach to air dehumidification involves lowering thetemperature setpoint of the HVAC system. This approach causes the HVACsystem to operate for longer periods of time than if the temperaturesetpoint of the HVAC system were set to a higher temperature. Thisapproach serves to reduce both the temperature and humidity of theconditioned air. However, this approach results in over-cooling of theconditioned air, which over-cooling often results in occupantdiscomfort. Additionally, consequent extended run times cause the HVACsystem to consume more energy, which leads to higher utility costs.Another air dehumidification approach involves re-heating of air leavingan evaporator coil.

In HVAC systems having a variable-speed compressor, the compressor speedmay be modulated with the cooling load. In an effort to maintain adesirable S/T ratio, a speed of an indoor circulation fan may also beadjusted with the compressor speed. When the HVAC system is operating inthe cooling mode, the speed of the indoor circulation fan is adjustedsuch that there is approximately 400 cubic feet per minute (“CFM”) peractual ton of cooling. When the HVAC system is operating in thedehumidification mode, the speed of the indoor circulation fan isadjusted such that there is approximately 200 CFM per actual ton ofcooling. As used herein, “actual ton” refers to an actual operatingcompressor tonnage. For example, in a two-compressor system having a3-Ton compressor and a 5-Ton compressor, the actual tonnage is 3 Tonsduring periods when only the 3-Ton compressor is operating. If the CFMper actual ton ratio is too high, the S/T ratio rises and limits theability of the HVAC system to remove humidity from the enclosed space.In practice, however, this can be difficult to accomplish as mechanicallimitations of the indoor circulation fan establish a minimum possibleCFM. Additionally, very low CFM results in poor air distribution withinthe enclosed space. In various embodiments, a minimum rated speed of theindoor circulation fan is established by a manufacturer of the indoorcirculation fan.

FIG. 1 illustrates an HVAC system 100. In a typical embodiment, the HVACsystem 100 is a networked HVAC system that is configured to conditionair via, for example, heating, cooling, humidifying, or dehumidifyingair within an enclosed space 101. In a typical embodiment, the enclosedspace 101 is, for example, a house, an office building, a warehouse, andthe like. Thus, the HVAC system 100 can be a residential system or acommercial system such as, for example, a roof top system. For exemplaryillustration, the HVAC system 100 as illustrated in FIG. 1 includesvarious components; however, in other embodiments, the HVAC system 100may include additional components that are not illustrated but typicallyincluded within HVAC systems.

The HVAC system 100 includes a variable-speed circulation fan 110, a gasheat 120, electric heat 122 typically associated with the variable-speedcirculation fan 110, and an evaporator coil 130, also typicallyassociated with the variable-speed circulation fan 110. Thevariable-speed circulation fan 110, the gas heat 120, the electric heat122, and the evaporator coil 130 are collectively referred to as an“indoor unit” 148. In a typical embodiment, the indoor unit 148 islocated within, or in close proximity to, the enclosed space 101. TheHVAC system 100 also includes a variable-speed compressor 140 and anassociated condenser coil 142, which are typically referred to as an“outdoor unit” 144. In various embodiments, the outdoor unit 144 is, forexample, a rooftop unit or a ground-level unit. The variable-speedcompressor 140 and the associated condenser coil 142 are connected to anassociated evaporator coil 130 by a refrigerant line 146. In a typicalembodiment, the variable-speed compressor 140 is, for example, asingle-stage compressor or a multi-stage compressor. The variable-speedcirculation fan 110, sometimes referred to as a blower, is configured tooperate at different capacities (i.e., variable motor speeds) tocirculate air through the HVAC system 100, whereby the circulated air isconditioned and supplied to the enclosed space 101.

Still referring to FIG. 1, the HVAC system 100 includes an HVACcontroller 150 that is configured to control operation of the variouscomponents of the HVAC system 100 such as, for example, thevariable-speed circulation fan 110, the gas heat 120, the electric heat122, and the variable-speed compressor 140 to regulate the environmentof the enclosed space 101. In some embodiments, the HVAC system 100 canbe a zoned system. In such embodiments, the HVAC system 100 includes azone controller 180, dampers 185, and a plurality of environment sensors160. In a typical embodiment, the HVAC controller 150 cooperates withthe zone controller 180 and the dampers 185 to regulate the environmentof the enclosed space 101.

The HVAC controller 150 may be an integrated controller or a distributedcontroller that directs operation of the HVAC system 100. In a typicalembodiment, the HVAC controller 150 includes an interface to receive,for example, thermostat calls, temperature setpoints, blower controlsignals, environmental conditions, and operating mode status for variouszones of the HVAC system 100. For example, in a typical embodiment, theenvironmental conditions may include indoor temperature and relativehumidity of the enclosed space 101. In a typical embodiment, the HVACcontroller 150 also includes a processor and a memory to directoperation of the HVAC system 100 including, for example, a speed of thevariable-speed circulation fan 110.

Still referring to FIG. 1, in some embodiments, the plurality ofenvironment sensors 160 are associated with the HVAC controller 150 andalso optionally associated with a user interface 170. The plurality ofenvironment sensors 160 provide environmental information within a zoneor zones of the enclosed space 101 such as, for example, temperature andhumidity of the enclosed space 101 to the HVAC controller 150. Theplurality of environment sensors 160 may also send the environmentalinformation to a display of the user interface 170. In some embodiments,the user interface 170 provides additional functions such as, forexample, operational, diagnostic, status message display, and a visualinterface that allows at least one of an installer, a user, a supportentity, and a service provider to perform actions with respect to theHVAC system 100. In some embodiments, the user interface 170 is, forexample, a thermostat of the HVAC system 100. In other embodiments, theuser interface 170 is associated with at least one sensor of theplurality of environment sensors 160 to determine the environmentalcondition information and communicate that information to the user. Theuser interface 170 may also include a display, buttons, a microphone, aspeaker, or other components to communicate with the user. Additionally,the user interface 170 may include a processor and memory that isconfigured to receive user-determined parameters such as, for example, arelative humidity of the enclosed space 101, and calculate operationalparameters of the HVAC system 100 as disclosed herein.

In a typical embodiment, the HVAC system 100 is configured tocommunicate with a plurality of devices such as, for example, amonitoring device 156, a communication device 155, and the like. In atypical embodiment, the monitoring device 156 is not part of the HVACsystem. For example, the monitoring device 156 is a server or computerof a third party such as, for example, a manufacturer, a support entity,a service provider, and the like. In other embodiments, the monitoringdevice 156 is located at an office of, for example, the manufacturer,the support entity, the service provider, and the like.

In a typical embodiment, the communication device 155 is a non-HVACdevice having a primary function that is not associated with HVACsystems. For example, non-HVAC devices include mobile-computing devicesthat are configured to interact with the HVAC system 100 to monitor andmodify at least some of the operating parameters of the HVAC system 100.Mobile computing devices may be, for example, a personal computer (e.g.,desktop or laptop), a tablet computer, a mobile device (e.g., smartphone), and the like. In a typical embodiment, the communication device155 includes at least one processor, memory and a user interface, suchas a display. One skilled in the art will also understand that thecommunication device 155 disclosed herein includes other components thatare typically included in such devices including, for example, a powersupply, a communications interface, and the like.

The zone controller 180 is configured to manage movement of conditionedair to designated zones of the enclosed space 101. Each of thedesignated zones include at least one conditioning or demand unit suchas, for example, the gas heat 120 and at least one user interface 170such as, for example, the thermostat. The zone-controlled HVAC system100 allows the user to independently control the temperature in thedesignated zones. In a typical embodiment, the zone controller 180operates electronic dampers 185 to control air flow to the zones of theenclosed space 101.

In some embodiments, a data bus 190, which in the illustrated embodimentis a serial bus, couples various components of the HVAC system 100together such that data is communicated therebetween. In a typicalembodiment, the data bus 190 may include, for example, any combinationof hardware, software embedded in a computer readable medium, or encodedlogic incorporated in hardware or otherwise stored (e.g., firmware) tocouple components of the HVAC system 100 to each other. As an exampleand not by way of limitation, the data bus 190 may include anAccelerated Graphics Port (AGP) or other graphics bus, a Controller AreaNetwork (CAN) bus, a front-side bus (FSB), a HYPERTRANSPORT (HT)interconnect, an INFINIBAND interconnect, a low-pin-count (LPC) bus, amemory bus, a Micro Channel Architecture (MCA) bus, a PeripheralComponent interconnect (PCI) bus, a PCI-Express (PCI-X) bus, a serialadvanced technology attachment (SATA) bus, a Video Electronics StandardsAssociation local (VLB) bus, or any other suitable bus or a combinationof two or more of these. In various embodiments, the data bus 190 mayinclude any number, type, or configuration of data buses 190, whereappropriate. In particular embodiments, one or more data buses 190(which may each include an address bus and a data bus) may couple theHVAC controller 150 to other components of the HVAC system 100. In otherembodiments, connections between various components of the HVAC system100 are wired. For example, conventional cable and contacts may be usedto couple the HVAC controller 150 to the various components. In someembodiments, a wireless connection is employed to provide at least someof the connections between components of the HVAC system such as, forexample, a connection between the HVAC controller 150 and thevariable-speed circulation fan 110 or the plurality of environmentsensors 160.

FIG. 2 is a schematic diagram of the exemplary HVAC system 100 with asuction pressure sensor 220. For illustrative purposes, FIG. 2 will bedescribed herein relative to FIG. 1. The HVAC system 100 includes theevaporator coil 130, the condenser coil 142, the variable-speedcompressor 140, and a metering device 202. In a typical embodiment, themetering device 202 is, for example, a thermostatic expansion valve or athrottling valve. The evaporator coil 130 is fluidly coupled to thevariable-speed compressor 140 via a suction line 204. The variable-speedcompressor 140 is fluidly coupled to the condenser coil 142 via adischarge line 206. In a typical embodiment, the variable-speedcompressor 140 is a variable-speed compressor. The condenser coil 142 isfluidly coupled to the metering device 202 via a liquid line 208.

Still referring to FIG. 2, during operation, low-pressure,low-temperature refrigerant is circulated through the evaporator coil130. The refrigerant is initially in a liquid/vapor state. In a typicalembodiment, the refrigerant is, for example, R-22, R-134a, R-410A,R-744, or any other suitable type of refrigerant as dictated by designrequirements. Air from within the enclosed space 101, which is typicallywarmer than the refrigerant, is circulated around the evaporator coil130 by the variable-speed circulation fan 110. In a typical embodiment,the refrigerant begins to boil after absorbing heat from the air andchanges state to a low-pressure, low-temperature, super-heated vaporrefrigerant. Saturated vapor, saturated liquid, and saturated fluidrefer to a thermodynamic state where a liquid and its vapor exist inapproximate equilibrium with each other. Super-heated fluid andsuper-heated vapor refer to a thermodynamic state where a vapor isheated above a saturation temperature of the vapor. Sub-cooled fluid andsub-cooled liquid refers to a thermodynamic state where a liquid iscooled below the saturation temperature of the liquid.

The low-pressure, low-temperature, super-heated vapor refrigerant isintroduced into the variable-speed compressor 140 via the suction line204. In a typical embodiment, the variable-speed compressor 140increases the pressure of the low-pressure, low-temperature,super-heated vapor refrigerant and, by operation of the ideal gas law,also increases the temperature of the low-pressure, low-temperature,super-heated vapor refrigerant to form a high-pressure,high-temperature, superheated vapor refrigerant. The high-pressure,high-temperature, superheated vapor refrigerant leaves thevariable-speed compressor 140 via the discharge line 206 and enters athree-way valve 262. When operating in a cooling mode, the three-wayvalve 262 directs that high-temperature, high-pressure, superheatedvapor refrigerant to the condenser coil 142.

When operating in a dehumidification mode, the three-way valve 262diverts at least a portion of the high-pressure, high-temperature,superheated vapor refrigerant into a re-heat feed line 264. The re-heatfeed line 264 directs the high-pressure, high-temperature, superheatedvapor refrigerant to a re-heat coil 266. In a typical embodiment, there-heat coil 266 is positioned in a supply air duct 256 downwind fromthe evaporator coil 130. The re-heat coil 266 facilitates transfer of aportion of the heat stored in the high-pressure, high-temperature,superheated vapor refrigerant to air moving through the supply air duct256 thereby heating the air in the supply air duct 256. If thehigh-pressure, high-temperature, superheated vapor refrigerant iswarmer, more heat can be transferred to the air in the supply air duct256 thereby causing a temperature of the air in the supply air duct 256to be closer to a temperature of air in a return air duct 254. Afterleaving the re-heat coil 266, the high-pressure, high-temperature,superheated vapor refrigerant travels through a re-heat return line 268and enters the condenser coil 142.

Outside air is circulated around the condenser coil 142 by a condenserfan 210. The outside air is typically cooler than the high-pressure,high-temperature, superheated vapor refrigerant present in the condensercoil 142. Thus, heat is transferred from the high-pressure,high-temperature, superheated vapor refrigerant to the outside air.Removal of heat from the high-pressure, high-temperature, superheatedvapor refrigerant causes the high-pressure, high-temperature,superheated vapor refrigerant to condense and change from a vapor stateto a high-pressure, high-temperature, sub-cooled liquid state. Thehigh-pressure, high-temperature, sub-cooled liquid refrigerant leavesthe condenser coil 142 via the liquid line 208 and enters the meteringdevice 202.

In the metering device 202, the pressure of the high-pressure,high-temperature, sub-cooled liquid refrigerant is abruptly reduced. Invarious embodiments where the metering device 202 is, for example, athermostatic expansion valve, the metering device 202 reduces thepressure of the high-pressure, high-temperature, sub-cooled liquidrefrigerant by regulating an amount of refrigerant that travels to theevaporator coil 130. Abrupt reduction of the pressure of thehigh-pressure, high-temperature, sub-cooled liquid refrigerant causessudden, rapid, evaporation of a portion of the high-pressure,high-temperature, sub-cooled liquid refrigerant, commonly known as“flash evaporation.” The flash evaporation lowers the temperature of theresulting liquid/vapor refrigerant mixture to a temperature lower than atemperature of the air in the enclosed space 101. The liquid/vaporrefrigerant mixture leaves the metering device 202 and returns to theevaporator coil 130.

Still referring to FIG. 2, the HVAC controller 150 is operativelycoupled to the variable-speed circulation fan 110 and to thevariable-speed compressor 140. A pressure sensor 220 is arranged tomeasure refrigerant pressure within the suction line 204. In variousembodiments, the pressure sensor 220 is, for example, a pressuretransducer or other appropriate device. In various embodiments, a speedof the variable-speed compressor 140 may be adjusted to correspond tochanging cooling loads. In such embodiments, the HVAC controller 150 mayadjust a speed of the variable-speed circulation fan 110 relative to aspeed of the variable-speed compressor 140. By way of example, inoperation, a decrease in the speed of the variable-speed compressor 140is detected by the HVAC controller 150. The HVAC controller 150 thensignals the variable-speed circulation fan 110 to reduce speed. Incertain conditions, however, reduction of the speed of thevariable-speed circulation fan 110 is constrained by mechanicallimitations of the variable-speed circulation fan 110. Additionally, lowspeeds of the variable-speed circulation fan can result in ineffectiveair distribution throughout the enclosed space 101.

FIG. 3 is a schematic diagram of a re-circulation duct 302. Forillustrative purposes, FIG. 2 will be described herein relative to FIGS.1-2. The re-circulation duct 302 is disposed so as to fluidly couple thesupply air duct 256 to the return air duct 254. A damper 304 is disposedin the re-circulation duct 302. During operation, the damper 304 ismovable between an open position, which allows air to pass from thesupply air duct 256, through the re-circulation duct 302, to the returnair duct 254, and a closed position, which does not allow passage of airthrough the re-circulation duct 302. In various embodiments, the damper304 is electrically coupled to the HVAC controller 150 and moves betweenthe open position and the closed position responsive to a signal fromthe HVAC controller 150.

Still referring to FIG. 3, during operation of the HVAC system 100, thepressure sensor 220 monitors refrigerant pressure in the suction line204 (referred to herein as “suction pressure”) and transmits a signalcorresponding to the suction pressure to the HVAC controller 150. If thepressure sensor 220 detects a suction pressure in the suction line 204above a pre-determined threshold, the HVAC controller 150 transmits asignal to the damper 304 directing the damper 304 to move from theclosed position to the open position thereby allowing air to flow fromthe supply air duct 256 to the return air duct 254 via there-circulation duct 302. In various embodiments, the pre-determinedsuction pressure threshold may be, for example, in the range ofapproximately 130 psi to approximately 150 psi when the HVAC system 100is operating in the cooling mode and in the range of approximately 110psi to approximately 130 psi when the HVAC system is operating in thedehumidification mode.

Still referring to FIG. 3, when the damper 304 is in the open position,a portion of air discharged from the variable-speed circulation fan 110travels through the re-circulation duct 302 to the return air duct 254and is not discharged to the enclosed space 101 via the supply air duct256. Moving the damper 304 to the open position reduces a volume of airsupplied to the enclosed space 101 and thus has an effect similar tothat of reducing a speed of the variable-speed circulation fan 110. Asused herein, the term “supply CFM” refers to a volume of air that isdelivered to the enclosed space via the supply air duct 256. The term“blower CFM” refers to a volume of air supplied by the variable-speedcirculation fan 110. The term “re-circulation CFM” refers to a volume ofair that travels through the re-circulation duct 302. Thus the, blowerCFM is the sum of the supply CFM and the re-circulation CFM.

FIGS. 4A-4D are graphs illustrating various performance parameters ofthe HVAC system 100 when the HVAC system 100 is operating in the coolingmode. FIG. 4A is a graph of suction pressure versus re-circulation CFM.FIG. 4A illustrates that, as re-circulation CFM increases, and supplyCFM thus decreases, suction pressure falls. FIG. 4B is a graph of latentcapacity versus re-circulation CFM. FIG. 4B illustrates that, asre-circulation CFM increases, latent capacity increases. Thus, theability of the HVAC system 100 to dehumidify air increases as there-circulation CFM increases. FIG. 4C is a graph of S/T ratio versusre-circulation CFM. FIG. 4C illustrates that, as re-circulation CFMincreases, S/T ratio decreases thereby indicating that the ability ofthe HVAC system 100 to dehumidify air increases as the re-circulationCFM increases. FIG. 4D is a graph of sensible capacity versusre-circulation CFM. FIG. 4D illustrates that, as re-circulation CFMincreases, sensible capacity decreases thereby indicating that theability of the HVAC system 100 to dehumidify air increases as there-circulation CFM increases.

FIGS. 5A-5D are graphs illustrating various performance parameters ofthe HVAC system 100 when the HVAC system 100 is operating in thedehumidification mode. FIG. 5A is a graph of suction pressure versusre-circulation CFM. FIG. 5A illustrates that, as re-circulation CFMincreases, and supply CFM thus decreases, suction pressure falls. FIG.5B is a graph of latent capacity versus re-circulation CFM. FIG. 5Billustrates that, as re-circulation CFM increases, latent capacityincreases. Thus, the ability of the HVAC system 100 to dehumidify airincreases as the re-circulation CFM increases. FIG. 5C is a graph of S/Tratio versus re-circulation CFM. FIG. 5C illustrates that, asre-circulation CFM increases, S/T ratio decreases thereby indicatingthat the ability of the HVAC system 100 to dehumidify air increases asthe re-circulation CFM increases. FIG. 5D is a graph of sensiblecapacity versus re-circulation CFM. FIG. 5D illustrates that, asre-circulation CFM increases, sensible capacity decreases therebyindicating that the ability of the HVAC system 100 to dehumidify airincreases as the re-circulation CFM increases.

FIG. 6 is a schematic diagram of an exemplary HVAC system 600 having adischarge air temperature sensor 602. For purposes of illustration, FIG.6 will be described herein relative to FIGS. 1-3. The HVAC system 600 issimilar in construction and operation to the HVAC system 100. The HVACsystem 600, however, omits the pressure sensor 220. The temperaturesensor 602 is positioned in the supply air duct 256 and thermallyexposed to air entering the enclosed space 101 via the supply air duct256. In various embodiments, the temperature sensor 602 is, for example,a thermometer, a thermocouple, a thermistor, or other appropriatedevice. In various embodiments, a speed of the variable-speed compressor140 may be adjusted to correspond to changing cooling loads. The HVACcontroller 150 adjusts a speed of the variable-speed circulation fan 110relative to a speed of the variable-speed compressor 140.

Still referring to FIG. 6, during operation of the HVAC system 600, thetemperature sensor 602 monitors temperature of air supplied to theenclosed space 101 via the supply air duct 256 (referred to herein as“discharge air temperature”) and transmits a signal corresponding to thesuction pressure to the HVAC controller 150. If the temperature sensor602 detects a discharge air temperature above a pre-determinedthreshold, the HVAC controller 150 transmits a signal to the damper 304directing the damper 304 to move from the closed position to the openposition thereby allowing air to flow from the supply air duct 256 tothe return air duct 254 via the re-circulation duct 302. In variousembodiments, the pre-determined discharge air temperature threshold maybe, for example, in the range of approximately 45 F to approximately 65F when the HVAC system 600 is operating in the cooling mode and in therange of approximately 40 F to approximately 60 F when the HVAC system600 is operating in the dehumidification mode.

Still referring to FIG. 6, when the damper 304 is in the open position,a portion of air discharged from the variable-speed circulation fan 110travels through the re-circulation duct 302 to the return air duct 254and is not discharged to the enclosed space 101 via the supply air duct256. Moving the damper 304 to the open position reduces a volume of airsupplied to the enclosed space 101 and thus has an effect similar tothat of reducing a speed of the variable-speed circulation fan 110.

FIG. 7A-7D are tables illustrating performance of the exemplary HVACsystem 600. FIG. 7A is a graph of discharge air temperature versusre-circulation CFM. FIG. 5A illustrates that, as re-circulation CFMincreases, and supply CFM thus decreases, discharge air temperaturefalls. FIG. 7B is a graph of latent capacity versus re-circulation CFM.FIG. 7B illustrates that, as re-circulation CFM increases, latentcapacity increases. Thus, the ability of the HVAC system 600 todehumidify air increases as the re-circulation CFM increases. FIG. 7C isa graph of S/T ratio versus re-circulation CFM. FIG. 7C illustratesthat, as re-circulation CFM increases, S/T ratio decreases therebyindicating that the ability of the HVAC system 600 to dehumidify airincreases as the re-circulation CFM increases. FIG. 7D is a graph ofsensible capacity versus re-circulation CFM. FIG. 7D illustrates that,as re-circulation CFM increases, sensible capacity decreases therebyindicating that the ability of the HVAC system 600 to dehumidify airincreases as the re-circulation CFM increases.

FIG. 8 is a flow diagram of a process 800 for utilizing a re-circulationduct 302. The process 800 starts at step 802. At step 804, adetermination is made if the speed of the variable-speed circulation fan110 is operating at a minimum rated speed. In various embodiments, aminimum rated speed of the variable-speed circulation fan 110 isestablished by a manufacturer of the variable-speed circulation fan 110.If it is determined at step 804 that the variable-speed circulation fan110 is not operating at a minimum rated speed, the process 800 proceedsto step 806. At step 806, the damper 304 is closed. If it is determinedat step 804 that the variable-speed circulation fan 110 is operating ata minimum rated speed, the process 800 proceeds to step 808. At step808, a determination if the HVAC system is operating in cooling mode andif the ratio of CFM per actual ton is greater than 400. As illustratedin FIGS. 2 and 6, an operating parameter of the HVAC system such as, forexample, the suction pressure or the discharge air temperature may beutilized as a proxy for CFM per actual ton ratio. If, at step 808, it isdetermined that the HVAC system is operating in cooling mode and theratio of CFM per actual ton is greater than 400, the process 800 proceedto step 812. At step 812, the damper 304 is opened in an effort increasethe latent capacity of the HVAC system. From step 812, the processreturns to step 804.

Still referring to FIG. 8, if, at step 808 it is determined that theHVAC system is either 1) not operating in cooling mode; or 2) the CFMper actual ton ratio is not greater than 400, the process 800 proceedsto step 814. At step 814, a determination is made if the HVAC system isoperating in dehumidification mode and if the CFM per actual ton ratiois greater than 200. If, at step 814, it is determined that the HVACsystem is operating in dehumidification mode and the CFM per actual tonratio is greater than 200, the process 800 proceeds to step 816. At step816, the damper 304 is opened in an effort increase the latent capacityof the HVAC system. From step 816, the process returns to step 804. If,at step 814 it is determined that the HVAC system is either 1) notoperating in dehumidification mode; or 2) the CFM per actual ton ratiois not greater than 200, the process 800 returns to step 804. In variousembodiments, hysteresis may be incorporated with at least one of steps806, 812, and 816 in order to prevent repeated opening and closing ofthe damper 304.

For purposes of this patent application, the term computer-readablestorage medium encompasses one or more tangible computer-readablestorage media possessing structures. As an example and not by way oflimitation, a computer-readable storage medium may include asemiconductor-based or other integrated circuit (IC) (such as, forexample, a field-programmable gate array (FPGA) or anapplication-specific IC (ASIC)), a hard disk, an HDD, a hybrid harddrive (HHD), an optical disc, an optical disc drive (ODD), amagneto-optical disc, a magneto-optical drive, a floppy disk, a floppydisk drive (FDD), magnetic tape, a holographic storage medium, asolid-state drive (SSD), a RAM-drive, a SECURE DIGITAL card, a SECUREDIGITAL drive, a flash memory card, a flash memory drive, or any othersuitable tangible computer-readable storage medium or a combination oftwo or more of these, where appropriate.

Particular embodiments may include one or more computer-readable storagemedia implementing any suitable storage. In particular embodiments, acomputer-readable storage medium implements one or more portions of theHVAC controller 150, one or more portions of the user interface 170, oneor more portions of the zone controller 180, or a combination of these,where appropriate. In particular embodiments, a computer-readablestorage medium implements RAM or ROM. In particular embodiments, acomputer-readable storage medium implements volatile or persistentmemory. In particular embodiments, one or more computer-readable storagemedia embody encoded software.

In this patent application, reference to encoded software may encompassone or more applications, bytecode, one or more computer programs, oneor more executables, one or more instructions, logic, machine code, oneor more scripts, or source code, and vice versa, where appropriate, thathave been stored or encoded in a computer-readable storage medium. Inparticular embodiments, encoded software includes one or moreapplication programming interfaces (APIs) stored or encoded in acomputer-readable storage medium. Particular embodiments may use anysuitable encoded software written or otherwise expressed in any suitableprogramming language or combination of programming languages stored orencoded in any suitable type or number of computer-readable storagemedia. In particular embodiments, encoded software may be expressed assource code or object code. In particular embodiments, encoded softwareis expressed in a higher-level programming language, such as, forexample, C, Python, Java, or a suitable extension thereof. In particularembodiments, encoded software is expressed in a lower-level programminglanguage, such as assembly language (or machine code). In particularembodiments, encoded software is expressed in JAVA. In particularembodiments, encoded software is expressed in Hyper Text Markup Language(HTML), Extensible Markup Language (XML), or other suitable markuplanguage.

The term “substantially” is defined as largely but not necessarilywholly what is specified (and includes what is specified; e.g.,substantially 90 degrees includes 90 degrees and substantially parallelincludes parallel), as understood by a person of ordinary skill in theart. In any disclosed embodiment, the terms “substantially,”“approximately,” “generally,” and “about” may be substituted with“within 10% of” what is specified.

Depending on the embodiment, certain acts, events, or functions of anyof the algorithms described herein can be performed in a differentsequence, can be added, merged, or left out altogether (e.g., not alldescribed acts or events are necessary for the practice of thealgorithms). Moreover, in certain embodiments, acts or events can beperformed concurrently, e.g., through multi-threaded processing,interrupt processing, or multiple processors or processor cores or onother parallel architectures, rather than sequentially. Although certaincomputer-implemented tasks are described as being performed by aparticular entity, other embodiments are possible in which these tasksare performed by a different entity.

Conditional language used herein, such as, among others, “can,” “might,”“may,” “e.g.,” and the like, unless specifically stated otherwise, orotherwise understood within the context as used, is generally intendedto convey that certain embodiments include, while other embodiments donot include, certain features, elements and/or states. Thus, suchconditional language is not generally intended to imply that features,elements and/or states are in any way required for one or moreembodiments or that one or more embodiments necessarily include logicfor deciding, with or without author input or prompting, whether thesefeatures, elements and/or states are included or are to be performed inany particular embodiment.

While the above detailed description has shown, described, and pointedout novel features as applied to various embodiments, it will beunderstood that various omissions, substitutions, and changes in theform and details of the devices or algorithms illustrated can be madewithout departing from the spirit of the disclosure. As will berecognized, the processes described herein can be embodied within a formthat does not provide all of the features and benefits set forth herein,as some features can be used or practiced separately from others. Thescope of protection is defined by the appended claims rather than by theforegoing description. All changes which come within the meaning andrange of equivalency of the claims are to be embraced within theirscope.

What is claimed is:
 1. A heating, ventilation, and air conditioning (HVAC) system comprising: an evaporator coil disposed between a supply air duct and a return air duct; a re-circulation duct, wherein the re-circulation duct fluidly couples the supply air duct and the return air duct; a damper disposed in the re-circulation duct, the damper being moveable between an open position and a closed position; a variable-speed circulation fan for circulating air around the evaporator coil; a variable-speed compressor fluidly coupled to the evaporator coil; a controller operatively coupled to the variable-speed compressor, the variable-speed circulation fan, and the damper; a pressure sensor disposed in a suction line between the evaporator coil and the variable-speed compressor, the pressure sensor being configured to measure a refrigerant pressure in the suction line; wherein the controller is configured to: determine if the variable-speed circulation fan is operating at a minimum speed and if a suction pressure measured by the pressure sensor is above a pre-determined threshold; responsive to a determination that the variable-speed circulation fan is operating at the minimum speed and the suction pressure is above the pre-determined threshold, signaling the damper to move to the open position; and responsive to a determination that the variable-speed circulation fan is not operating at the minimum speed or the suction pressure is below the pre-determined threshold, signaling the damper to move to the closed position.
 2. The HVAC system of claim 1, wherein the speed of the variable-speed circulation fan is modulated responsive to a speed of the variable-speed compressor.
 3. The HVAC system of claim 1, wherein moving the damper to the open position reduces a volume of air supplied via the supply air duct.
 4. The HVAC system of claim 1, wherein the HVAC system operates in at least one of a cooling mode and a dehumidification mode.
 5. The HVAC system of claim 4, wherein the pre-determined threshold is in the range of approximately 130 psi to approximately 150 psi when the HVAC system is operating in the cooling mode.
 6. The HVAC system of claim 4, wherein the pre-determined threshold is in the range of approximately 110 psi to approximately 130 psi when the HVAC system is operating in the dehumidification mode.
 7. The HVAC system of claim 1, wherein moving the damper to the open position increases latent capacity of the HVAC system.
 8. A heating, ventilation, and air conditioning (HVAC) system comprising: an evaporator coil disposed between a supply air duct and a return air duct; a re-circulation duct that fluidly couples the supply air duct and the return air duct; a damper disposed in the re-circulation duct, the damper being moveable between an open position and a closed position; a variable-speed circulation fan for circulating air around the evaporator coil; a variable-speed compressor fluidly coupled to the evaporator coil; a controller operatively coupled to the variable-speed compressor, the variable-speed circulation fan, and the damper; wherein the controller is configured to: determine if the variable-speed circulation fan is operating at a minimum speed; monitor an operating parameter of the HVAC system; determine if the operating parameter of the HVAC system exceeds a pre-determined threshold; responsive to a determination that the variable-speed circulation fan is operating at the minimum speed and the operating parameter exceeds the pre-determined threshold, signaling the damper to move to the open position; and wherein the operating parameter is a refrigerant pressure in a suction line.
 9. The HVAC system of claim 8, wherein the speed of the variable-speed circulation fan is modulated responsive to a speed of the variable-speed compressor.
 10. The HVAC system of claim 8, wherein moving the damper to the open position reduces a volume of air supplied via the supply air duct.
 11. The HVAC system of claim 8, wherein the HVAC system operates in at least one of a cooling mode and a dehumidification mode.
 12. The HVAC system of claim 8, wherein a pressure sensor is configured to monitor the operating parameter in the suction line.
 13. A method of utilizing a re-circulation duct, the method comprising: determining with an HVAC controller if a variable-speed circulation fan is operating at a minimum speed; monitoring an operating parameter of an HVAC system; determining if the operating parameter of the HVAC system exceeds a pre-determined threshold; responsive to a determination by the HVAC controller that the variable-speed circulation fan is operating at the minimum speed and the operating parameter exceeds the pre-determined threshold, signaling with the HVAC controller a damper disposed in a re-circulation duct that fluidly couples a supply air duct and a return air duct to move to an open position; and wherein the operating parameter is a refrigerant pressure in a suction line.
 14. The method of claim 13, wherein the monitoring the operating parameter includes monitoring the refrigerant pressure in the suction line with a pressure sensor.
 15. The method of claim 13, wherein the operating parameter is an air temperature in the supply air duct.
 16. The method of claim 15, wherein the monitoring the operating parameter includes monitoring the air temperature in the supply air duct with a temperature sensor.
 17. The method of claim 13, comprising diverting air from the supply air duct to the return air duct via the re-circulation duct responsive to opening the damper. 